Data are written to read/write from/to an optical disk using a writing waveform prepared using a reference waveform based on a unique writing parameter (hereinafter, also referred to as a writing strategy or WST) of the optical disk, so that writing quality can be maintained. A unique WST of an optical disk is determined through procedures of writing information to the optical disk using a reference writing strategy (reference WST), reproducing the written information to detect a length of a written mark or space, and correcting a writing waveform in a manner such that the difference between the detected length and a theoretical length by the reference WST. That is, a predetermined WST obtained in this way is determined as a unique WST of the optical disk.
In a typical method, it is assumed that a leading/trailing edge position of a writing signal generated for a writing test based on a WST is linearly related with a average leading/trailing edge position of a reproducing signal read from a written edge pattern, and the average leading/trailing edge position of the read reproducing signal is set to zero. FIG. 6 is a reproducing signal waveform diagram for explaining parameters of a WST according to a typical method. In FIG. 6, xij, yli, and yjm denote variations of leading/trailing edge positions of an I-mark and a j-mark of a writing signal from currently set positions.
μljm denotes a current average leading edge position of a reproducing signal specified by combinations of an l-mark, an i-space, a j-mark, and an m-space. dij, clij, and bljm denote influence coefficients indicating the influences of the variations xij, yli, and yjm on the average leading edge position of the reproducing signal. rljim denotes probabilities of combinations of the l-mark, the i-space, the j-mark, and the m-space. Each average leading/trailing edge position of the reproducing signal can be made to be zero by using solutions of an equation: 0=Σlmrlijm(μlijm+dijxij+clijyli+bijmyjm).
In the above-described method, averages of differences between a writing signal and a reproducing signal generated by reading each written pattern are calculated at each leading edge position and each trailing edge position, and each of the averages is set to zero. Therefore, since an adjustable WST is limited to each leading/trailing edge position of a writing signal, the extendibility is lacking, and the writing quality and precision are limited.
In another method disclosed in Japanese Patent Application Laid-Open No. 2007-287229, a system controller 32 writes test data in a test region of an optical disk 10 using various WSTs and evaluates reproducing quality of the test data to select an optimal WST. For this, the system controller 32 calculates an evaluation value Hst for each WST by using the equation: Hst=AEb2+BWe2+C(Peb−Pmb)2 where Eb denotes an error rate minimum, We denotes a writing power range for obtaining a threshold error rate, Peb denotes a writing power level for obtaining a target value β, and Pmb denotes a writing power level for obtaining a minimal error rate. Then, a WST resulting in a minimal Hst is treated as an optimal WST, which is applied in the same way to jitter.
In the disclosed method, test data are written according to previously prepared WSTs, and the written data are read and measured for selecting best one of the previously prepared WSTs. Therefore, more writing tests are necessary for improving precision. In this case, for determining a WST, a larger writing test region should be provided on an optical disk, and test evaluation time increases.